docs: def append(self, p_todo, p_string):
          """
          Appends a text to the todo, specified by its number.
          The todo will be parsed again, such that tags and projects in de
          appended string are processed.
          """
          if len(p_string) > 0:
              new_text = p_todo.source() + ' ' + p_string
              p_todo.set_source_text(new_text)
              self._update_todo_ids()
              self.dirty = True